1. Field of the Invention
The present invention relates hydrophilic crosslinked porous polymeric materials and methods for preparing such materials. In one embodiment, the hydrophilic polymeric materials are produced in the form of microbeads.
2. Description of the Related Art
Crosslinked porous polymeric materials are disclosed in U.S. Pat. No. 4,522,953 (Barby et al., issued Jun. 11, 1985). The disclosed polymeric materials are produced by polymerization of "water-in-oil" emulsions having a relatively high ratio of water to oil, typically on the order of 70% or more. These emulsions are termed "high internal phase emulsions" and are known in the art as "HIPEs." The HIPEs disclosed in U.S. Pat. No. 4,522,953 comprise an oil continuous phase including a monomer and a crosslinker and an aqueous discontinuous phase. Such emulsions are prepared by subjecting the combined oil and water phases to agitation in the presence of an emulsifier. Polymers are produced from the resultant emulsion by heating or other means. The polymers are then washed to remove any unpolymerized emulsion components.
The disclosed porous polymers have rigid structures characterized by cavities interconnected by pores in the cavity walls. By choosing appropriate component and process conditions, HIPE polymers with void volumes of 70% or more can be achieved. These materials thus have a very high capacity for absorbing and retaining liquids.
Although the polymerization of oil-phase monomer and crosslinker described by Barby and others produced hydrophobic HIPE polymers, such polymers could be made hydrophilic by conjugation with hydrophilic groups. For instance, U.S. Pat. No. 4,536,521 (Haq, issued Aug. 20, 1985) discloses that HIPE polymers can be sulfonated to produce a sulfonated polymeric material that exhibits a high capacity for absorption of ionic solutions. Other functionalized HIPE polymers prepared by a similar process have been disclosed in U.S. Pat. No. 4,611,014 (Jomes et al., issued Sep. 9, 1986) and U.S. Pat. No. 4,612,334 (Jones et al., Sep. 16, 1986).
Initially processes for making HIPE polymers produced blocks of polymeric material the size and shape of the vessel used for polymerization. A problem with producing HIPE polymers in block form was that it is very difficult to wash unpolymerized emulsion components out of blocks of low density, highly absorbent material. Attempts to address this problem by grinding the blocks into particles were unsatisfactory because both the drying and milling processes are costly, and there is a limit to the size of the particles produced by milling.
The state of the art was significantly advanced by producing HIPE polymers in the form of microbeads, as disclosed in U.S. Pat. No. 5,583,162 (Li et al., issued Dec. 10, 1996) and International Application No. PCT/US95/06879 (WO 95/33553, published Dec. 14, 1995). The disclosed "HIPE microbeads" are produced by polymerization of a suspension of HIPE droplets. A key feature of one polymerization method described in International Application No. PCT/US95/06879 was the use of a film-forming stabilizer to reduce leakage of the discontinuous phase of the HIPE from HIPE droplets into the medium in which the droplets were dispersed. This aspect of the method helped preserve the HIPE-like structure of the droplets upon conversion to microbeads.
It is desirable to produce hydrophilic polymeric materials having a HIPE-like structure directly, i.e., by polymerizing hydrophilic monomers. The polymerization of hydrophilic monomers in an "oil-in-water-in-oil" emulsion is described in U.S. Pat. No. 4,742,086 (Masamizu, issued May 3, 1988). According to this process, an oil-in-water emulsion is prepared and added to a hydrophobic dispersing medium. The process is disclosed as useful for preparing polymeric beads from oil-in-water emulsions that are less than 70% oil (internal) phase. Thus, the resultant hydrophilic beads are not HIPE microbeads.